MRI is a powerful diagnostic imaging modality providing highly detailed images throughout the human body without the use of ionizing radiation and is used to scan more than 30 million patients every year in the United States. Organization for Economic Co-operation and Development (2012). Magnetic resonance imaging (MRI) exams, total. Health: Key Tables from OECD, No. 46.
Each year about 600,000 pacemakers are implanted and in 2002 there were about three (3) million patients worldwide living with implanted pacemakers. Wood M A, Ellenbogen K A. Cardiology patient pages. Cardiac pacemakers from the patient's perspective. Circulation, 2002; 105:2136-2138.
Significantly higher numbers of patients are implanted with other active and passive medical devices. Studies have shown that up to 75% of patients with implanted pacemakers will require Magnetic Resonance Imaging (MRI) within the lifetime of their device. Kalin R, Stanton M S. Current clinical issues for MRI scanning of pacemaker and defibrillator patients. Pacing Clin Electrophysiol 2005; 28:326-8. Significantly higher numbers of patients with other types of implanted medical devices are expected to require MRI.
Exposure of an implanted medical device to the electromagnetic fields generated by an MRI system can damage surrounding tissue, reduce the effectiveness of the device, promote migration of the device, or cause tissue necrosis. For example, one of the safety concerns for patients with any implanted device is RF induced heating during MRI. When electrically conductive materials are subjected to oscillating magnetic fields, electric currents are induced, which cause heating and increase the temperature in the surrounding tissue, potentially reducing the effectiveness of the device or causing tissue necrosis.
Currently, these safety concerns cause more than one million patients with pacemakers, implanted cardioverter defibrillators (ICDs) and other implanted devices to be denied access to MRI procedures on an annual basis worldwide. Martin E T. Can cardiac pacemakers and magnetic resonance imaging systems co-exist? Eur Heart J, 2005; 26(4):325-327. Although a limited number of MRI-conditional devices, such as pacemakers, are becoming available, and previous studies have demonstrated successful 1.5 T MRI procedures for some patients with pacemakers, the vast majority of implanted devices are contraindicated for MRI. MRI-conditional devices represent only a small fraction of the devices in use worldwide, and millions of patients have existing implanted devices that are not registered as safe for exposure to conventional MRI fields. Wood M A, Ellenbogen K A. Cardiology patient pages. Cardiac pacemakers from the patient's perspective. Circulation, 2002; 105:2136-2138. Further, the move towards higher magnetic field strengths (e.g., 3.0 T and 7.0 T) makes MR safety a growing concern.
The Food and Drug Administration (FDA) currently requires medical device manufacturers to evaluate the MR safety of their medical devices. However, the currently accepted standard test method (ASTM F2182) for measuring RF induced heating does not accurately predict in vivo tissue damage or account for physiologic heat transfer mechanisms. Current attempts to extrapolate in vitro behavior to in vivo environments overly simplify the relationship between applied RF energy and device heating to an extent that safe exposure to MRI operation cannot be reliably ascertained. Similarly, tests to measure the effect of the Gradient coil and permanent magnetic field of an MRI machine are not representative of in vivo application, and the collected data cannot be safely extrapolated to humans.
Conventional methods that estimate RF induced heating through correlations with Specific Absorption Rate (SAR) and temperature rise or, when extended for in vivo, apply transfer function approaches that are arbitrary abstractions of the response of real device geometries in real body habitus. Current predictions of SAR are accomplished using Finite Difference Time Domain (FDTD) that has significant inherent restrictions on the accurate representation of the complexity of the geometry and orientation of the implanted device. Thus, these approaches rely on simplifying assumptions that restrict integration of patient and device specific information and thus patient specific precision medicine approaches are not possible with the traditional methodologies.
The current clinical practice paradigm for assessing the MR safety risks of patients with implanted devices is reliant on safety data collected under conditions that misrepresent clinical application. Simplified in vitro safety tests do not account for patient-specific information such as device placement and orientation, the effect of local blood flow in the body, the type of MRI scanner hardware, or MRI electromagnetic fields. These parameters significantly impact the potential for interaction between the MRI's electromagnetic field and the implanted device and thus have to be included in any patient specific risk assessment.
The current test standards used to determine MR safety are overly conservative because they do not represent in vivo physiological conditions, and do not account for patient specific factors such as body habitus and device geometry. As a result, patients with pacemakers, ICD's, and other implanted devices may be inappropriately withheld from critical MRI scans because their devices are considered unsafe. Thus, there is a need for a method to determine the how an implant will react to an MRI scan which overcomes the limitations of the prior art.